stages of implantation ..apposition , adhesion and invasion finally decidualization

1. Stages of Implantation
Dr.Pavithra.A
Pharm.d

2. Apposition
 Apposition is the first connection between blastocyst and endometrium .
 During this stage, receptor-ligand interactions are critical.
 The blastocyst enters into the uterus, rolling freely over the endometrium expressing
adhesion molecules such as L- selectin .
 Selectins play an important role in this step ensuring the rolling and tethering of
the blastocyst.
 The human embryo needs to align to the receptive endometrium with a specific
inner cell mass orientation to ensure a proper apposition.
 These selectins mediate the apposition of the blastocyst into the uterine
epithelium interacting with L- selectin ligands which are mainly detected on
pinopodes where blastocyst adhesion is initiated

3. Contd...
 The implanting embryo also encounters a glycocalyx associated with the luminal
epithelium that contains different adhesion molecules.
 One of them is MUC-1, identified as an anti-adhesion molecule .
 The purpose of MUC-1 at this stage is to prevent the blastocyst from binding to
an area with poor chances of implantation.
 Its expression increases just before implantation to prevent the embryo
attaching in the wrong location

4. Adhesion/Attachment
 Removal of the pre-existing layer of mucins is necessary for blastocyst adhesion.
 During the apposition stage, the presence of the blastocyst promotes the increase of
levels of MUC-1 in the luminal epithelium, but, at the beginning of the adhesion
phase, the blastocyst induces the cleavage of MUC-1 at the implantation site to
promote successful attachment
 Several chemokines and cytokines are essential during the process of adhesion.
 One of their functions is to attract the blastocyst to the location of implantation.
 The most relevant cytokine for implantation and the most studied is LIF

5.  Leukemia inhibitory factor (LIF), play an important role during
implantation by supporting the embryo-endometrial interactions
 In the endometrium, expression of LIF reaches maximal levels during the
mid-secretory phase, during which the endometrium is under the
influence of progesterone
 Adhesion molecules such as integrins, are also necessary to attach the
blastocyst to the pinopodes to ensure a firm implantation
 Among these integrins, the heterodimer V3 is crucial for endometrial
recognition during the adhesion.
 It is expressed in the human trophoblast cells and uterine luminal
epithelium during implantation and participates in endometrial recognition

6. Invasion/Penetration
 During invasion or penetration, trophoblast cells from the blastocyst penetrate the
endometrial epithelium invading the underlying endometrial stroma with the
purpose of reaching maternal blood vessels
 Trophoblast cells start developing thin folds, named invadopodia, that grow
between
adjacent endometrial epithelial cells.
 They are intended for the degradation of the basement membrane, allowing the
trophoblast cells to spread into the endometrial stroma
 Trophoblast cells proliferate and differentiate into inner cytotrophoblast and
outer syncyotiotrophoblast

7.  The syncyotiotrophoblast invades the luminal epithelium, which is called
syncytialization
 In humans, the embedding of the blastocyst within the stroma is completed 8 days after
ovulation occurs and the entry site is then covered with fibrin
 The syncytiotrophoblasts fluid-filled spaces separate by trabeculae, appearing to
transform the syncyotiotrophoblast into a spongy material .The trabeculae are arranged
radially,and cytotrophoblastic cells proliferate within the trabeculae, forming a primary
chorionic villus.
 Over time, the primary villi grow and branch into secondary and tertiary villi. This
process is known as placentation

8. Decidualization
 Once implantation occurs and the embryo breaches the luminal
epithelium, the stromal cells surrounding the implanting embryo
transform into a decidua by a process called decidualization
• Cellular Composition of Decidual Stroma
 uterine stromal cells-main type of decidua cell
 Macrophages, lymphocytes and decidual leukocytes also play a
role during decidualization
 Decidual leukocytes -play a role in providing maternal immune
tolerance, they also contribute to decidual remodeling during
pregnancy

9.  Among these leukocytes, the uterine natural killers (uNK) are the most involved in
the maternal immune tolerance (70% of the decidual immune cells).
 They are present in the human endometrium across the cycle and become activated
and dramatically increase during decidualization
 An important feature of decidua is its function in controlling trophoblast invasion.
 During implantation, the decidua differentiates into different regions:
 1.) decidual basalis underneath the implantation site,
 2.) decidua parietalis adjacent to basalis
 3.) decidual secretory endometrium which remains similar to the pre- decidualized
endometrium

10.  Immune cells also play a role during the formation of the decidua.
 Leukocytes - infiltrate into the endometrium in response to hormonal signaling
occurring during the invasion
 Macrophages, growth factors and cytokines are expressed by uNK cells that facilitate
and control the invasion of the trophoblast cells, while promoting vascular
transformation
 The cytotrophoblast cells develop anchoring systems that promote the interaction
with the decidual stromal cells, glands and the maternal immune system.
 The trophoblast moves towards the maternal blood vessels under the control of
hormonal and cytokine signaling .
 Angiogenesis and vasculogenesis change the decidua with the purpose of initiating
the development of the placenta and to coordinate an independent (embryo-mother),
vascular system

11. Role of Notch during Decidualization
 In the human endometrium, the Notch1-3 receptors are located in epithelial and
stromal cells.
 Ligands Jagged1 (Jag1) and Delta-like 4 (DLL4) are primarily present in
epithelial cells
 The expression of Notch1 and its target, α -SMA, are enhanced by CG from the
implanting blastocyst and progesterone(CG- Chorionic gonadotropin)
 Progesterone also plays an important role in the Notch1 regulation.

12.  Progesterone, along with CG activate Notch1, which promotes α -SMA expression
and inhibits apoptosis in stromal cells by initiating their differentiation into decidual
cells.
 Notch signaling promotes proliferation, dierentiation and apoptosis in the
primate endometrium
 Notch1 plays a key role during the transformation of stromal fibroblast into
decidual cells.
 Any failure in Notch1 signaling could result in subsequent impaired
decidualization compromising pregnancy.